Galen Marek was took by Darth Vader as a child to become "The Ultimate Sith Lord". He was sent on many tasks some easy some hard.

Then Obi-Wan Kenobi saw him and thought to himself," This kid has potential.Maybe I should persuade him to join the Light Side and become a Jedi."

Galen Marek was persuaded to become a Jedi Warrior and his mission was simple:Kill Vader.